The prior art satellite systems comprise a fixed position antenna mounted on a satellite in geo-stationary orbit. Typically in such systems, the antenna is fixed so that the spot or the "footprint" of the signal beam covers a portion of the globe that is "visible" to the satellite.
The ground level strength of the signal transmitted by the antenna in these prior art systems is very weak as a consequence of the great distance to the geo-stationary orbits. This creates a problem, even for the highest powered geo-stationary satellites, since the ground level signal is too weak to allow for in-building coverage. For example, for a geo-stationary satellite with a radiated power of 400 watts, a selective call device using prior art selective call signaling protocol at a bit rate of 512 bits per second has been observed to have only 2 dB of signaling margin. That is, the average signal strength on the ground is only 2 dB above the minimum level required to activate a selective call device. Thus, considering that most buildings attenuate (radio frequency) RF signals by 20 dB (decibel), these systems could not be used to signal selective call users inside buildings.
Some satellite communication systems are in closer orbits and consequently achieve significantly higher ground level signal strengths by steering the antenna beam to cover different geographical areas in a repetitive pattern to achieve wide area coverage. These lower orbits necessarily result in significant relative velocities between an observer on the ground and the satellite resulting in a variable Doppler frequency shift and variable transmission delays which derogate receiver sensitivity making signal acquisition extremely difficult.
Thus, what is needed is a method and apparatus to minimize the Doppler frequency shifts and to equalize path delays associated with a satellite communication system.